The invention relates to an electroluminescent device comprising at least one display element which comprises at least one electroluminescent layer of organic material, which is sandwiched between two conductor layers, which device is configured as a diode, which device generates light when said layer is subjected to an electric field.
The invention further relates to a method of manufacturing such an electroluminescent device.
An electroluminescent (EL) device is a device which, while making use of the phenomenon of electroluminescence, emits light when the device is suitably connected to a power supply. If the light emission originates in an organic material, said device is referred to as an organic electroluminescent device. An organic EL device can be used, inter alia, as a thin light source having a large luminous surface area, such as a backlight for a liquid crystal display or a watch. An organic EL device can also be used as a display if the EL device comprises a number of EL elements, which may or may not be independently addressable.
The use of organic layers as an EL layer in an EL element is known. A class of said known organic layers generally comprise a conjugated, luminescent compound. Said compound may be a low-molecular dye, such as a coumarin, or a high-molecular compound, such as a poly(phenylenevinylene). The EL element also comprises two electrodes, which are in contact with the organic layer. By applying a suitable voltage, the negative electrode, i.e. the cathode, will inject electrons and the positive electrode, i.e. the anode, will inject holes. The recombination of these electrons and holes may generate light. If the EL element is in the form of a stack of layers, at least one of the electrodes should be transparent to the light to be emitted. A known transparent electrode material for the anode is, for example, indium tin oxide (ITO). Known cathode materials are, inter alia, Al, Ba, Yb, Mg:Ag, Li:Al or Ca. Known anode materials are, in addition to ITO, for example, gold and platinum. If necessary, the EL element may comprise additional organic layers, for example, of an oxadiazole or a tertiary amine, which serve to improve the charge transport or the charge injection.
An EL device of the type mentioned in the opening paragraph is disclosed in EP-A-0 893 939, which describes an electroluminescent display device comprising an organic electroluminescent element formed on a substrate and an organic compound layer group sandwiched between cathodes and anodes. The organic compound layer group has laminated electroluminescent functional layers formed of at least one kind of organic compound. The organic electroluminescent display device further includes an airtight case that encloses the organic electroluminescent element with a space formed between the airtight case itself and the organic electroluminescent element and isolates the organic electroluminescent element from outside air.
As disclosed in EP-A-0 893 939, there may be a small risk of dust adhesion on the organic EL element. Since the organic EL functional layer has a very small thickness of a submicron order, a short circuit due to the dust is liable to occur between the transparent electrode (anode) and the back electrode (cathode). The conventional countermeasure for preventing short circuits are washing and smoothing of the surface of the substrate. However, as it is impossible to perfectly clean the substrate surface, there still remains possibility of occurrence of short circuits.
Also the influence of moisture, oxygen or the like, when permeated into the organic layer via a defect in, for example, the cathode layer, may cause deterioration of the organic electroluminescent functional layer.
In order to reduce the risk of a short circuit, EP-A-0 893 939 employs a filler gas filling the space within the airtight case. Said filler gas includes at least one kind of combustion-supporting gas. The combustion-supporting gas oxidizes the cathode material of a portion surrounding a short circuit to form an insulator, and thereby suppresses the occurrence of a short circuit. A combustion-supporting gas is a gas-oxidizing agent that does not burn itself, but helps combustion of other materials. Known combustion-supporting gasses are, e.g. oxygen, dinitrogen oxide, ozone, chlorine, nitrogen monoxide, fluorine, nitrogen trifluoride, oxygen difluoride, etc.
The necessary encapsultation of a filler gas implies strong restrictions on future architectures of organic electroluminescent devices.
The invention aims to provide a reliable organic electroluminescent device as described in the first paragraph where the risk for failure before the end of its lifetime, is strongly reduced.
To this end the organic electroluminescent device is characterized in that the leakage current is less than 10 xcexcA per cm2 electroluminescent surface area when said layer is subjected to a reverse bias in the range of 1-10 volts. Said leakage current of less than 10 xcexcA per cm2 electroluminescent surface area is the leakage current through the area of the electroluminescent layer which, in operation, may emits light. Preferably, the instabilities in the leakage current, referred to as noise, have a noise level less than 0.02, in particular less than 0.017. To obtain this measure for the noise, the measurements are normalized with Inorm as will be explained below. It has been found that devices that meet this criterion have a low risk for failure before the end of their lifetime.
The invention is based on the recognition that not only a leakage current, but also an unstable leakage current may be a precursor for the occurrence of an early failure such as a short circuit. The leakage current is the current through the electroluminescent device, which is configured as a diode, when said diode is operated in reverse bias. In order to characterize the instabilities in the leakage current, a noise level is determined as follows:
a. A predetermined, reverse voltage (here 1 V) is set on the electroluminescent device, configured as a diode, during a predetermined time interval (here 10 seconds).
b. The leakage current of an electroluminescent device is sampled (here every 0.1 seconds, yielding 100 measurements).
c. The NOISE at the certain reverse voltage is calculated using the following equation:
NOISE=((|xcex94I(t=2.5 s)|+|xcex94I(t=2.6 s)|+ . . . +|xcex94I(t=9.8 s)|)/74)/Inorm
Were |xcex94I(t=2.5 s)| is positive value of the difference of the leakage current measured at time t=2.5 seconds and measured at time t=2.6 seconds after the reverse voltage is set. Inorm is determined as follows:
Inorm=(I(t=2.5 s)+I(t=2.6 s)+ . . . +I(t=9.9 s))/75
The first 25 points of the measurement are discarded due to the effect of local heating on the resistance of the local leakage path leading to a change of the current.
d. The reverse voltage is switched off for 10 seconds.
e. Subsequently procedure steps a to d are repeated for reverse voltages 2, 3, 4, 5, 6, 7, 8, 7, 6, 5, 4, 3, 2, and 1 V on the electroluminescent device, configured as a diode. The NOISE is determined for every reverse voltage.
f. The maximum value of the NOISE measurements is the noise level of the device is less than 0.02.
The normalization procedure enables the comparison the noise levels of different devices despite differences in leakage current.
In the device in accordance with the invention the leakage current is preferably less than 0.5 xcexcA per cm2 electroluminescent surface area when said layer is subjected to 4 volts in reverse bias. The instabilities in the leakage current, referred to as noise, preferably have a noise level less than 0.006. Of said devices, less than 1% shows an early failure.
Measurements yielded a clear correlation between the height of the noise level and the occurrence of an early failure for a collection of samples. However for this measurement the processing of the devices was made optimal. Leakage currents are extremely low and the correlation found indicates that the leak path responsible for the unstable behavior is the same as the leak path which responsible for most of the leakage current. As the instabilities will be of the order of the leakage current of that particular leakage path the normalization procedure will yield reliable results. The noise levels of different devices can be compared despite differences in leakage current (the reason why the normalization procedure is used). However, in a realistic production situation, processing will never be optimal. The leakage current will be an order of magnitude higher and the contribution of the current flowing through the unstable leak path to the total leakage current is only a fraction of the total leakage current. Consequently, noise level measurements on devices made under less optimal processing conditions may be quite unreliable.
The leakage current is approximately proportional to the square of the reverse voltage. Other leakage currents, background leakage currents which do not originate from the electroluminescent layer, show an approximately linear dependency between the leakage current and the reverse voltage. In conclusion, the leakage current can be distinguished from the other leakage currents by studying the non-linear component of the leakage current when the device is subjected to a reverse bias in the range from 1-10 volts. In particular, when the determination of a reliable NOISE-value is hindered by large background leakage currents, it is preferred to normalize with V2 in stead of Inorm. Here V2 is the square of the reverse voltage during the measurement. In the device according to the invention it is preferred that the instabilities in the leakage current have a noise level of less than 8.33 nA/V2. The normalization with V2 yields a number, referred to as NOISE(V), which is proportional to the NOISE as defined above. It was found that a NOISE of 0.020 corresponds approximately with a NOISE(V) of 8.33 nA/V2, and a NOISE of 0.006 corresponds approximately with a NOISE(V) of 2.50 nA/V2. The NOISE(V) is particularly useful for electroluminescent devices having a large background current, such as matrix displays.
Surprisingly, a pre-conditioning procedure applied to the devices made under less optimal processing conditions shows a remarkable effect on the leakage currents and on the instabilities in the leakage current.
To this end a method of manufacturing an electroluminescent device is characterized in that the device is subjected to a reverse field of at least 100 V per micrometer electroluminescent layer thickness.
When the reverse electric field is established a leakage current starts to flow. For some devices the leakage current is very high or will increase in time, and such devices must be rejected as being unreliable. Other devices will show a decrease of this leakage current in time, which suggests a kind of repairing mechanism. Local pathways for the leakage current seem to be made inactive or to be destroyed. The device is subjected to the reverse electric field until the leakage current is reduced to less than 10% of the leakage current at the beginning of the pre-conditioning procedure.
The exact voltage used to establish the reverse electric field used in the procedure is not too important, or to put it differently, there is quite a broad range of voltages were the procedure behaves in a similar way. This is important as the thickness variations of the electroluminescent layer for different devices allowed for by the presently used manufacturing process are reasonably large (10%). The electric field is equal to the applied voltage divided by the thickness.
If the reverse electric field over the organic layer is too high this will lead to damage. In other words, the reverse electric field must not result in an electrical breakdown of the layer of organic material. On the other hand when a too reverse electric field is applied, the pre-conditioning procedure will not work. When using a relatively low reverse electric field strength of 100 V/xcexcm, said pre-conditioning procedure may be repeated to obtain the desired effect.
Preferably, the reverse electric field is at least 170 volts per micrometer electroluminescent layer thickness. A pre-conditioning procedure using these high reverse electric fields yielded less rejected devices after the first pre-conditioning cycle, suggesting a means of increasing the yield of the production process.
Preferably, a method of manufacturing an electroluminescent device of the invention is characterized in that the device is subjected to a reverse voltage in the range from 10 to 15 volts. As the light emitting layer of the device is typically 100 nm thick, the reverse electric field is in the range of approximately 100-150 volts per micrometer electroluminescent layer thickness. Because, the leakage current predominantly originates from local pathways, the electric voltage over these local pathways is predominantly the driving force for this pre-conditioning procedure. The electric field over the electroluminescent layer influences the pre-conditioning procedure to a lesser extend.
The method according to the invention may also be advantageous for organic electronics in general and organic diodes, also non-light-emitting, in particular. Said organic electronic devices may also suffer from leakage currents and/or leakage current instabilities, with the risk of a short circuit. The reliability of said organic electronic devices may also be increased by using the pre-conditioning process and/or the noise level measurements.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.